The invention relates to a method for operating an internal combustion engine which is operated in at least one engine operation range in a substantially homogeneous self-ignited manner and in at least a second engine operation range in a substantially homogeneous spark-ignited manner, with a changeover occurring during engine operation between homogeneous self-ignited operation and homogeneous spark-ignited operation depending on the load and vice-versa, as well as an internal combustion engine for performing the method.
The combustion of a lean fuel/air mixture which is ignited by self-ignition leads to the advantage that due to the homogeneous concentration and temperature distribution extremely low emission values are achieved for NOx and exhaust particulates. This method is known in the English-speaking countries as xe2x80x9cHCCIxe2x80x9d method (Homogeneous Charge Compression Ignition). The low content of NOx exhaust gas in the HCCI method is obtained by the fact that the combustion commences at a plurality of ignition locations, as a result of which the combustion occurs with a relatively low combustion temperature. Gasoline shows considerable advantages over diesel fuel for the HCCI method due to its very low willingness for auto-ignition and the lower boiling range of between approx. 30xc2x0 and 190xc2x0. The compression ratio can be increased in this case, as in a diesel engine, to values of approx. 15 to 17. Since the precise time of the ignition can fixed only at low effective medium pressure in the desired manner shortly before the upper dead center, the achievable effective mean pressure is limited in the HCCI method in a disadvantageous manner to the partial-load range, as is explained in the publication xe2x80x9cAn Experimental Study on Premixed-Charge Compression Ignition Gasoline Enginexe2x80x9d, Taro Aoyama et al., SAE 960081.
DE 199 27 479 A1 describes a method for operating an internal combustion engine operated with gasoline which provides that the internal combustion engine is self-ignited in a homogeneous way below a predetermined limit value of the effective medium pressure and is operated in a spark-ignited manner above the limit value. In this way the advantages of the HCCI method can be used without its disadvantages.
The HCCI operating mode is provided primarily to cover the low- and partial-load fields, whereas the spark-ignited engine operation is used for the higher partial load and the full load. In the dynamic operation of an internal combustion engine it is unavoidable to change between the two operating modes during the operation of the engine without producing serious losses in the torque delivery in the transition phases.
From EP 1 085 192 A2 an internal combustion engine is known which is operated in the medium partial-load range in a homogeneous self-ignited manner and in the upper partial-load range, in the full-load range as well as the lowermost partial-load range in a homogeneous spark-ignited fashion. In the range of the changeover from spark-ignited to self-ignited operation the quantity of recirculated exhaust gas is increased in order to ensure a secure self-ignition. Conversely, during the changeover from self-ignited to spark-ignited operation the quantity of recirculated exhaust gas is reduced on time in order to prevent knocking phenomena. During the change from one operating mode to another a transitional operation is performed while fuel is injected directly into the combustion chamber and a stratified charge is produced. A stable and even combustion during the change of the operating times is to be thus achieved. The transitional operation has a disadvantageous effect on the response characteristic and the emissions.
In homogeneous spark-ignition operation, the filling is usually realized by throttling the supply of fresh air by means of a throttling member in the fresh-air line or by throttling the fresh-air supply by means of a variable valve drive mechanism. If one examines a constant speed/load load in both operating modes it will be seen that at the same torque delivery in the homogeneous spark-ignited operation substantially more energy is bound in the exhaust gas than in HCCI operation. This fact is of decisive importance when the operating mode is to be changed. In addition, the homogeneous spark-ignition mode is operated with a substantially lower exhaust gas recirculation than the homogeneous spark-ignition operation.
During HCCI operation, the filling of the cylinder is not throttled. Instead, the engine is driven in this operating mode in an unthrottled manner. On changing the operating point, the filling composition changes which is composed of the mass of fresh-air, the recirculated mass of exhaust gas of the last combustion and the injected fuel mass. In this operating mode the cylinder charge is conditioned with the help of the recirculated exhaust gases in such a way that a self-ignition process can occur. The compression ignition occurs via the temperature increase of the fuel-air mixture during the compression phase in which the cylinder charge is compressed to a minimum residual volume.
When the internal combustion engine is operated in both operating modes in a stationary manner, the required mass of fresh air and the mass of recirculated exhaust gas is read out from engine characteristics map records which were determined under stationary conditions and set via adjusting the phase adjuster, the variable valve drive and/or the electric throttle member. Since the temperature conditions, and in particular the exhaust gas temperatures, deviate to an extremely high extent from the stationary values in the transitional phases, it is necessary to correct the mass of fresh air and the recirculated or compressed residual gas mass in these transitions in such a way that no excessive strain occurs in the internal combustion engine in the transient phase.
The choice as to which internal combustion engine is most suitable for which operating point is determined by a superset operating mode coordinator. The critical aspect in the change of the operating mode is the transition from the homogeneous spark-ignited operation to the homogeneous self-ignited operation because already in the first cycle after the changeover the cylinder charge needs to be composed in such a way that a combustion with self-ignition occurs at the right crank position. A very decisive variable which influences the position of the combustion in homogeneous self-ignition operation is the mixing temperature of the cylinder charge. If the mixing temperature of the cylinder charge is too high, the combustion occurs too early and the occurrence of very high pressure increases must be expected. This is accompanied by an enormous strain on the engine and a loud combustion noise. If on the other hand the mixing temperature of the cylinder charge is too low, the combustion occurs either too late or the conditions for the self-ignition are not achieved and a combustion misfire occurs. This constellation is to be avoided in order to avoid any noticeable torque drop at the clutch and on the other hand to keep the emissions of the internal combustion engine low.
It is the object of the present invention to improve a method of the kind mentioned above in such a way that an engine-protecting, low-emission and delay-free transition is possible between the engine operation ranges with homogeneous spark ignition and homogeneous self-ignition.
This is achieved in accordance with the invention in such a way that the homogeneous self-ignition operation is changed into the homogeneous spark-ignition operation and/or the homogeneous spark-ignition operation into the homogeneous self-ignition operation in a transitionless way from one cycle to the next cycle. An exhaust gas mass which is reduced with respect to the requirements for a stationary homogeneous self-ignition operation is recirculated or left in the cylinder during the changeover process from the homogeneous spark-ignition operation to the homogeneous self-ignition operation. By reducing the recirculated exhaust gas mass during the changeover phase it is possible to reduce the mixing temperature of the cylinder filling and subsequently the combustion state can be kept constant.
Due to the high exhaust gas temperatures in homogeneous spark-ignition operation, the recirculated exhaust gas mass is reduced for the first cycles of the homogeneous self-ignition operation because otherwise a combustion at a too early time would occur otherwise in which very high pressure gradients would have to expected in the combustion chamber. The adjustment of the recirculated exhaust gas mass can occur in such a way that for the purpose of reducing the recirculated exhaust gas mass the opening of at least one exhaust valve, which opening is performed for the purpose of internal exhaust gas recirculation during the inlet phase, is reduced during the intake phase and thus the mixing temperature of the filling is reduced. The adjustment of the recirculated exhaust gas mass can also occur as an alternative thereto via a combination of intake and exhaust control time by extending the intake control time and reducing the exhaust control time in order to compensate a filling loss. A further possibility is that for reducing the remaining exhaust gas mass the exhaust control time is increased during the exhaust phase. As a result, a lower exhaust gas mass remains in the cylinder at the end of the exhaust phase.
In order to enable a smooth transition from the spark-ignition operation to self-ignition operation it is advantageous when after the changeover-induced reduction the recirculated exhaust gas mass is increased continuously or discontinuously according to the requirements for homogeneous self-ignition operation depending on the combustion cycles that have occurred since the changeover. In an especially advantageous embodiment it is provided that at the time of the changeover a cycle counter is started which adds up the combustion cycles since the last homogeneous spark-ignited combustion and that the recirculated exhaust gas mass is determined as a function of the cycle count.
During the transition from homogeneous self-ignition operation to homogeneous spark-ignition operation, the valve control times can be changed abruptly from one cycle to the next cycle. In order to enable the performance of a compensation of the torque during the changeover phase, the ignition point can be adjusted during the changeover in the direction towards the upper dead center of the ignition.
The first engine operation range is allocated to homogeneous self-ignition and the second engine operation range to homogeneous spark-ignition.
The method is preferably employed with an internal combustion engine operated with gasoline in which it is provided that the internal combustion engine comprises a fully variable valve actuation device and a device for the controlled recirculation of exhaust gas, so that it is possible to change in a transitionless manner between homogeneous self-ignition operation and homogeneous spark-ignition operation and/or between homogeneous spark-ignition operation and homogeneous self-ignition operation.